Various types of devices have been developed for implantation into the human body to provide various types of health-related therapies, diagnostics, and/or monitoring. Examples of such devices, generally known as implantable medical devices (IMDs), include various types of cardiac rhythm management devices, such as cardiac pacemakers, cardioverter/defibrillators, cardiomyostimulators. Some IMDs may also be configured as various physiological stimulators including, for example, nerve, muscle, and deep brain stimulators, or as various types of physiological monitors, or drug delivery systems, just to name a few. Some IMDs include varying amounts of electronic memory that may be used to store not only device operating and control software, but to store various types of patient- and device-related data. In addition, some of these same IMDs may include signal processing and telemetry circuitry, which allows some or all of the data stored in the memory to be transmitted to a remote computer network or other communication node, and/or the device to receive and store data transmitted to it remotely from a computer network or other communication node.
The above-mentioned devices are typically implanted in a convenient location usually under the skin of the user and in the vicinity of the one or more major arteries or veins. One or more electrical leads connected to the pacemaker are inserted into or on the heart of the user, usually through a convenient vein or artery. The ends of the leads typically include one or more electrodes that are placed in contact with the walls or surface of one or more chambers of the heart, depending upon the particular therapies deemed appropriate for the user. The IMD may include a plurality of semiconductor switches, such as MOS transistors, that are selectively transitioned between conductive and non-conductive states to supply one or more cardiac pacing pulses to one or more of the electrodes, to stimulate the heart in one of several ways, again depending upon the particular therapy being delivered. The leads may also be configured to simultaneously sense the physiologic signals provided by the heart to determine when to deliver a therapeutic pulse to the heart, and the nature of the pulse, e.g., a pacing pulse or a defibrillation shock.
The type and number of electrodes associated with a lead may vary depending, for example, on the particular cardiac therapies being delivered. For example, the leads in many IMDs include at least a tip electrode and a ring electrode, and the leads in some IMDs additionally include a coil electrode. In this latter instance, the MOS pacing switches in the IMD may be configured to supply a pacing pulse either between the tip electrode and the ring electrode or between the ring electrode and the coil electrode. In such instances, it is possible that the control voltages supplied to the MOS transistor gates may need to be larger in magnitude than the highest magnitude IMD system supply voltages in order to maintain the MOS transistor in its conductive state or its non-conductive state.
More specifically, if the pacing switches are implemented using NMOS transistors, the control voltage may need to be a value that is more positive than the positive-most IMD system supply voltage in order to maintain the NMOS transistor in its conductive state, and the control voltage may need to be a value that is more negative than the negative-most IMD system supply voltage in order to maintain the NMOS transistor in its non-conductive state. Alternatively, if the pacing switches are implemented using PMOS transistors, the control voltage may need to be a value that is more negative than the negative-most IMD system supply voltage in order to maintain the PMOS transistor in its conductive state, and the control voltage may need to be a value that is more positive than the positive-most IMD system supply voltage in order to maintain the PMOS transistor in its non-conductive state.
The above-described MOS transistor gate voltages could be realized by including additional components within the IMD control circuitry, and/or by including additional regulated power supplies within the system. However, this could lead to undesirable increases in device implementation costs.
Hence, there is a need for a system and method of supplying MOS semiconductor pacing switch gate voltages that have sufficient magnitudes to keep the switches in a commanded conductive state without having to provide additional components within the IMD control circuitry and/or additional regulated power supplies within the system. The present invention addresses at least this need.